This invention relates to novel electroluminescent compositions and devices with enhanced performance. The compositions are a mixture of isomeric aromatic amine (IAA) compounds. The mixture can be vacuum evaporated to form amorphous thin films. The electroluminescent (EL) devices incorporate the novel compositions.
Organic EL devices are generally composed of a single or multiple layers of organic materials sandwiched between transparent and metallic electrodes. Organic EL devices are attractive owing to the requirement for low driving voltage and the fact that they are generally simple and relatively easy and inexpensive to fabricate. Furthermore, the light generated by organic EL devices are sufficient for use in a variety of ambient light conditions (from little or no ambient light to bright ambient light). There has been an increased interest in developing energy-efficient flat-panel displays based on organic EL devices primarily because of their potential as an emissive display technology which offers unrestricted viewing angles and high luminescence output at low operating voltages. Because of these advantages, organic EL have a potential application in full color flat emissive displays as well as displays in small products, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. However, despite recent advances that have been accomplished in EL device design and fabrication, a number of the current available EL device performance characteristics are not effectively suited for most practical applications. These characteristics include short serviceable life, low EL efficiency, and the rectification of all these performance deficiencies represents one formidable challenge in EL device research and development. Accordingly, one of the features of the present invention in embodiments thereof is to provide organic EL devices which provide extended device life span and excellent EL efficiency.
Prior art organic EL devices have been constructed from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices require excitation voltages on the order of 100 volts or greater. Subsequent modifications of the device structure through incorporation of additional layers, such as charge injecting and charge transport layers, have led to performance improvement. Illustrative examples of EL devices have been disclosed in publications by Tang et al. in J. Appl. Phys. vol. 65, pp. 3610 to 3616 (1989) and Saito et al. in Mol. Cryst. Liq. Cryst. vol. 253, pp. 125 to 132 (1994), the disclosures of which are totally incorporated herein by reference.
An EL device with an organic dual layer structure comprises one layer adjacent to the anode supporting hole injection and transport, and another layer adjacent to the cathode supporting electron injection and transport. The recombination of charge carriers and subsequent emission of light occurs in one of the layers near the interface between the two layers. Optionally, a fluorescent material capable of emitting light in response to recombination of holes and electrons can be added to one of the layers. In another configuration, an EL device can comprise three separate layers, a hole transport layer, an emission layer, and an electron transport layer, which are laminated in sequence and are sandwiched as a whole between an anode and a cathode.
Specifically, U.S. Pat. No. 4,356,429 discloses an EL device formed of an organic luminescent medium consisting of a hole transporting layer and an electron transporting layer, wherein the hole transporting layer is comprised of a porphyrinic compound. Further, in U.S. Pat. No. 4,539,507 there was substituted an aromatic tertiary amine layer for the hole transporting porphorinic layer. Illustrative examples of the aromatic tertiary amine compounds disclosed in the U.S. Pat. No. 4,539,507 are triphenylamines, such as N,N,N-triphenylamine and N,N,N-tri-p-tolylamine, those containing at least two aromatic tertiary amine moieties such as 1,1-bis(4-di-tolylaminophenyl)cyclohexane, and tetraaryidiamines such as N,Nxe2x80x2-bis(3-methylphenyl)-N,Nxe2x80x2-diphenyl-4,4xe2x80x2-diaminobiphenyl, N,N,Nxe2x80x2Nxe2x80x2-tetra-p-tolyl-4,4xe2x80x2-diaminobiphenyl. Also, of interest with respect to EL devices are U.S. Pat. Nos. 5,487,953 and 5,554,450.
Although recent performance improvements in organic EL devices have suggested a potential for widespread use, most practical applications require limited operation voltage or light output variance over an extended period of time. While hole transport materials comprised of certain aromatic tertiary amines are generally known to facilitate hole injection and hole transport processes which lead to improved device performance for organic EL devices. The thermal and morphological instability of these materials as the hole injecting and transport thin film layers has resulted in the short operational lifetime and poor durability of the organic EL devices for practical applications.
Many current organic EL devices possess limited operational lifetime, particularly at a high temperature of, for example, above 40xc2x0 C. One aspect which significantly affects the performance of organic EL devices is the thermal and morphological stability of the organic layers comprising the devices. These layers are amorphous thin films formed by vacuum deposition technique. The transition of an organic thin film from an amorphous state to a crystalline state can result in a physical or morphological change in the thin film. The integrity of organic EL devices with multi-layer structures is sensitive to this morphological change primarily because the charge carriers transport characteristics are substantially affected by the microscopic structures of the organic layers. This also will led the organic EL device tends to grow dark spots and shot circuit.
There is a need to use materials that can form amorphous thin film without recrystallization for long storage and operational life especially at elevated temperature. Polymer materials may have such advantage. However, it is not only difficult to synthesis and purify such materials, but also polymeric materials can not form thin film by vacuum deposition techniques. There is also a need to develop materials which are readily accessible synthetically, and which can be prepared in high yields and with excellent electronic purity. Another need resides in the provision of materials used in EL devices which are capable of forming thermally and morphologically stable thin films by for example, vacuum deposition techniques. A still further need is the preparation of new hole transport materials suitable for organic EL device applications, and which materials possess excellent hole transport characteristics, enabling the EL devices to operate at low voltages. These and other needs can be achievable with the EL devices of the present invention in embodiments thereof.
It is an object of the present invention to provide organic EL devices with many of the advantages illustrated herein.
Another object of the present invention is to provide organic EL devices with enhanced thermal stability and operational stability.
In another object of the present invention there are provided improved EL devices which exhibit high electroluminescence efficiency at relatively low operating voltages.
Yet in another object of the present invention there are provided improved EL devices comprising an anode, a cathode, and an organic electroluminescent medium is between the anode and the cathode, wherein the organic electroluminescent medium has at least one layer con a mixture of isomeric aromatic amines as the hole injection or transport layer.
Another object of the present invention is the provision of a certain mixture of isomeric aromatic amine compounds for EL devices, which compounds have a high glass transition temperature and a preparation process of the mixture of isomeric aromatic amines.
A further object of the present invention is the provis ion of EL devices with a mixture of isomeric aromatic amines which possess excellent hole injecting and transporting capability and superior thermal stability. This mixture of isomeric aromatic amines can be readily vacuum deposited as thin films for use as hole injection and or transport components in EL devices.
In embodiments, the present invention relates to layered organic EL devices comprising an anode, a cathode and between the anode and the cathode, an organic luminescent medium comprised of a hole injecting and hole transporting zone or layer containing a mixture of isomeric aromatic amines and an electron injecting and transporting zone or layer. These EL devices possess a number of advantages such as improved thermal stability, long device life, high electroluminescence efficiency, superior hole injecting and electron transporting characteristics. The devices can be readily fabricated using vacuum deposition techniques. The invention EL devices exhibit improved thermal and operational stability, and excellent device durability at elevated temperatures for both operation and storage.
In embodiments, the present invention relates to EL devices that are comprised in the following order: a supporting substrate, an anode, a hole injecting and transporting zone or layer, an electron injecting and transporting zone or layer, and a cathode, and wherein the hole injecting and transporting zone is comprised of a mixture of isomeric aromatic amines represented by the following Formula 1,
[(A1)a+(A2)b+ - - - +(An)x]xe2x80x83xe2x80x83(1) 
wherein:
A1, A2, . . . and An represent each individual components of the mixture of isomeric aromatic amines; These isomeric amines contain at least 24 carbon atoms and have a general molecular formula: 
wherein:
Ar1 is an aryl group or substituted aryl group containing at least 18 carbon atoms; Ar2 and Ar3 are individual aryl groups or substituted aryl groups containing at least 6 carbon atoms;
Each individual component (A1, A2, . . . and An) in the mixture has the same molecular formula. The difference of the individual component is the sequences of their atoms, or the point of attachment of substituents;
a, b, - - - and x are the ratio of each of the components A1, A2, . . . An in the mixture, range from 0 to 100%. The sum of a, b, - - - x is 1;
There are at least two components in a mixture of aromatic amine isomers.